MB is blowing PS fuse

The fuse on my 90W opus blew after plugging the opus back in after unplugging it while the PC was running and would not shut down.

Measured voltage on 90W opus with nothing connected--getting 12V on the
3.3V rail!

Subsequent tests inside have revealed that the Opus fried the MB--it will not come on with a regular desktop power supply with which it formerly worked with flawlessly. Instead it starts to smoke. Aparently the north and south bridges and other highly-integrated chips running on 3.3V logic have been destroyed.